Datacenter
Data center raised floor vs slab design field guide
The traditional raised access floor with an under-floor plenum versus slab-on-grade with everything overhead, why high density is shifting the choice, and the consequences that follow it.
Direct answer
A raised access floor sets IT racks on pedestals above the slab and uses the under-floor void as a plenum for cold air, power, and cabling. Slab-on-grade puts racks on the structural floor with cooling, power, and cabling overhead. High-density and AI halls now lean toward slab plus overhead, but the density and cooling design control the call.
Key takeaways
- Raised access floors set racks on pedestals above the slab and use the under-floor void as a plenum for cold air, power, and cabling.
- Slab-on-grade puts racks on structural concrete with cooling, power, and cabling overhead, and is now the default for high-density AI halls.
- Raised-floor plenum depth commonly runs 12 in to 36 in; published guidance puts the usable range near 6 in to 30 in, at least 18 in where airflow matters.
- AI racks with GPUs and in-rack liquid cooling routinely exceed 4,000 lb, and a flooded coolant distribution unit alone can weigh about three tons.
- Choose the floor on four numbers: density per rack, cooling method, wet rack weight with move-in rolling load, and building clear height; decide early because the floor drives every route.
The two floors, and the shift between them
A raised access floor sets the IT racks on a grid of removable panels carried on pedestals above the structural slab, and uses the gap underneath as a plenum. Slab-on-grade skips the raised floor: the racks sit on the structural concrete and the cooling, power, and cabling all run overhead. Both are in service today. New high-density and AI work is moving to slab plus overhead, and that shift is the reason this comparison matters now.
For roughly thirty years the raised floor was the default for a computer room. The under-floor void did three jobs at once. It was the cold-air supply plenum, the route for power, and the route for cabling. That fit a hall where each rack pulled a few kilowatts and air up through perforated tiles was enough to cool it. The density that broke the model is recent, and a lot of running floors still sit on raised access tile.
This guide is the design comparison: the floor decision and what it drives. It leans on two companions and does not repeat them. The structural side, the load ratings and the field load test, lives in the raised-floor load rating guide. The building zoning, the white space and gray space split, lives in the layout guide. Read this one for the choice between the two floors and the consequences that follow it.
What is a raised floor plenum?
A raised floor plenum is the sealed, pressurized void under an access floor that a computer room uses to deliver cold supply air. A CRAC or CRAH unit pushes conditioned air down into the void, the void holds it under a slight positive pressure, and perforated tiles or floor grilles set in the cold aisle let that air up into the equipment intakes. The plenum is the distribution path. The perforated tile is the register.
The same void carried more than air on the classic design. Power whips from floor PDUs and the structured cabling both ran under the tile, out of sight and out of the aisle. That is the picture most people still have of a data center. Lift a tile and find cold air, conduit, and a tangle of cable all sharing the space.
The pressurized-plenum idea is what made the raised floor worth its cost. You did not duct air to each rack. You pressurized one big void and opened a tile wherever you needed supply, which let the room change as the load moved. The airflow and the tile placement are covered by topic in the layout guide, and the structural side of the same floor is the load-rating guide. The point here is what the plenum did, and why a hall that no longer needs under-floor air loses the main reason it was built that way.
How a raised access floor is built
A raised access floor is a kit of parts, and each part has a job. Pedestals are the vertical columns, bonded to the slab and adjustable for height, that carry the load down. Stringers are the horizontal members that lock the pedestal heads into a grid and carry lateral and rolling load. The panels, the tiles, drop onto that grid, commonly 600 mm (about 24 in) square, in solid, perforated, or grated form. A ramp or a set of steps gets people and gear up to floor level at the room entry.
The height is the plenum depth, and it is a design number. Field plenums commonly run from about 12 in up to 36 in, with deeper voids on higher-airflow or higher-density halls. Published guidance puts the usable range from roughly 6 in to 30 in and recommends at least 18 in where airflow matters. Deeper is better for air and worse for ceiling height, which is a trade the building settles early.
The load rating, the grounding, and the field load test are their own subject, covered in the raised-floor load rating guide. The short version for this comparison: the floor is rated several ways, the rolling load during fit-out usually governs, and the rating only holds if the pedestals are plumb and bonded and the stringers are tight. Read that guide for the structural side. Here the construction matters because it is the cost and the weight limit that the slab approach removes.
The slab and overhead approach
Slab-on-grade puts the IT on the structural floor and lifts everything else overhead. There is no raised floor, no plenum, no under-floor void. The racks land on the concrete, leveled and anchored to it directly. The cold air comes from overhead supply, in-row units between the cabinets, or a contained aisle, not from a tile. The power runs overhead on busway. The cabling runs overhead on tray. Look up, not down.
The appeal is that the load path is short and honest. The rack weight goes straight into the slab, which is sized as building structure, so the point-load worry that shapes a raised-floor design mostly goes away. The cooling is delivered at or near the rack instead of being pushed through a shared void, which scales further as density climbs. And the overhead routes are visible and reachable without lifting a tile or working on your knees.
Slab is not new, but it was the exception for years and is now the default on high-density builds. The reason is that the things the plenum did best, distributing moderate-density air, matter less as the cooling moves to liquid and in-row, while the things the plenum did poorly, carrying heavy concentrated loads and staying sealed against leakage, matter more.
Why are data centers moving to slab?
Data centers are moving to slab plus overhead because high-density and AI loads need more cooling than a floor plenum can deliver, and because the slab handles the weight that comes with them. A pressurized under-floor plenum has a ceiling on how much air it can push up through tiles before the static pressure and the tile count run out. Once a rack pulls into the tens of kilowatts, the plenum cannot feed it, and the cooling moves to in-row, rear-door, or direct-to-chip liquid, none of which need the void.
Weight is the second driver. AI racks packed with GPUs and in-rack liquid cooling routinely run past 4,000 lb, and the support gear is heavier still. Putting that on a slab is a structural detail the building already handles. Putting it on a raised floor means heavier panels, more pedestals, and a tighter understructure, and the cost climbs fast. The load side is covered in the raised-floor load rating guide.
Flexibility and access close the case. Overhead busway and tray are visible, reachable, and easy to extend with a plug-in tap, where under-floor work means lifting tiles and crawling. Equinix and others have reported no compelling cost advantage for raised floor in new construction. Add it up and the floor that was the default for moderate density is the wrong default for a dense AI hall.
Air delivery: under-floor plenum vs overhead supply
The clearest difference between the two designs is how the cold air reaches the equipment. A raised floor delivers it from below: the plenum holds conditioned air under pressure and perforated tiles in the cold aisle let it up into the intakes. A slab delivers it from above or beside: overhead supply diffusers, in-row units set between cabinets, rear-door heat exchangers on the rack, or a fully contained aisle that pins the supply to the intakes.
Under-floor air is good at moderate, even density and gets harder to balance as the load concentrates. The pressure under the floor is not uniform. Tiles near the units can get too much air and tiles far away too little, and a high-density rack can pull more air than the tile in front of it can pass. Overhead and in-row put the cooling closer to the load and scale higher, which is why the dense rows go that way.
The airflow detail, the hot-aisle and cold-aisle arrangement and the containment, is covered by topic in the layout guide. The design point here is that the cooling method drives the floor. A hall built around under-floor air needs the raised floor. A hall built around liquid and in-row does not, and that single fact is moving the floor decision.
Cabling and power: under the floor vs overhead
Power and cabling went under the floor on the classic design and go overhead on the modern one, and the overhead trend started before the floor decision tipped. Under a raised floor, power arrived by whips from floor PDUs and the copper and fiber both ran in the plenum. Overhead, the power rides on busway, a continuous bus run with plug-in tap-off boxes, and the cabling rides on tray, commonly in layered runs with signal, power, and fiber kept apart.
Busway took over the dense floor for reasons that hold whether or not there is a raised floor under it. It runs overhead and leaves the cooling path clear, it is rated high enough for dense rows, and a rack feed can move or grow by plugging in a new tap instead of pulling a new conduit. TIA-942 lays out an overhead tray scheme for the structured cabling, with the layers separated and clearances held.
The under-floor problem the overhead trend solves is congestion. Cabling and power in the same void that is supposed to move air block the air, and the more cable that accumulates over the life of the hall, the worse the plenum cools. Pull the cabling and the power overhead and the void only has to move air, or the void goes away entirely with the slab.
Can a raised floor carry a high-density rack?
A raised floor can carry a high-density rack only if it is rated for it and the install is right, and that is a harder and more expensive thing to guarantee than putting the rack on a slab. The weight reaches the floor through a few small contact patches, the leveling feet at rest and the casters while the rack rolls in, and each is a concentrated and then a rolling load the panel and the pedestal have to take.
A slab takes that weight directly, because it is building structure sized for it. The point load that shapes a raised-floor design is mostly a non-issue on a slab. That is why the heaviest AI halls go slab: the floor question moves to the building structure under the IBC and ASCE 7, where it belongs, instead of to a panel rating.
The full load story, the concentrated, rolling, ultimate, and pedestal ratings, the way the rolling load during fit-out usually governs, and the field load test, is the subject of the raised-floor load rating guide. For the design choice, the rule is simple. If the rack weight and the move-in rolling load push past what a rated access floor carries economically, the slab wins.
Seismic anchoring on each floor
Seismic design is simpler on a slab, and on a raised floor it is one more system that has to be engineered and proven. On a slab the rack anchors straight to the structural concrete, with isolation or bracing as the code requires, a short and direct load path to ground. The structure that resists the earthquake is the building.
On a raised floor the lateral load has further to travel. The rack ties to the floor, the floor's pedestals and stringers have to carry the lateral and overturning load down to the slab, and the pedestal base-to-slab bond is what stops the column from tipping. A bolted-stringer understructure carries far more lateral load than a snap-on or stringerless system, which is why halls in seismic zones spec the bolted grid. Add seismic bracing for the floor itself and the cost and the coordination grow.
The seismic anchorage of the floor and the racks follows the IBC and the referenced ASCE 7 for the nonstructural components, and the specific bracing is project and jurisdiction specific. Confirm it against the structural drawings and the adopted code. The design point is that the slab removes a layer of seismic load path, and in a high-seismic region that is a real argument for it.
Plenum leakage and airflow management
The under-floor plenum leaks, and the leakage is the quiet tax on a raised-floor design. Every cable cutout, every gap around a pedestal, every unsealed tile edge bleeds conditioned air out of the void where you do not want it. The air that escapes through an open cutout under a rack does no cooling, and it drops the static pressure that feeds the tiles you do want.
The numbers are worse than people expect. Airflow studies have tied roughly half of a room's cooling capacity to cold air lost through unsealed cable openings in bad cases, and a count of two hundred unsealed openings has been equated to leaving ten floor tiles out. Brushed grommets sealing the cutouts have been measured cutting that bypass airflow by more than 90 percent. The management work is real and ongoing: seal the cutouts with grommets, blank the open rack U-spaces, and place the perforated tiles only where the load actually is.
This is maintenance the slab does not need, because there is no plenum to seal. Overhead supply with hot-aisle containment runs in a simpler pressure regime, and there is no void quietly leaking the air you paid to cool. The leakage problem is one of the strongest practical arguments against a raised floor on a hall that does not need under-floor air.
Liquid cooling and the floor
Liquid cooling changes the floor math, and it pushes toward slab. Direct-to-chip and immersion move the heat into a fluid loop at or in the rack, so the room no longer needs a pressurized void to push cold air at the equipment. The main reason the raised floor existed weakens the moment the cooling goes liquid.
Weight pushes the same way. A liquid-cooled rack is heavier than an air-cooled one, and the support gear is heavy on its own. A coolant distribution unit flooded with fluid can weigh on the order of three tons, and the manifolds and piping add more. A slab carries that without a second thought. A raised floor has to be rated for it, and the rating gets expensive. The weights and the load cases are covered in the load rating guide.
There is also a hazard the raised floor makes worse. Liquid cooling means fluid in or near the white space, and a leak over an open plenum runs down into the void where the power and the cabling live, out of sight, until the detection catches it or something faults. On a slab the floor is the catch surface and the leak is visible. The combination, no plenum needed, heavy gear, and the under-floor leak risk, is why new liquid-cooled halls are built on slab far more often than on raised floor.
Plenum depth, ceiling height, and the building
The raised floor costs you ceiling height, and the slab gives it back. A raised floor adds its plenum depth, commonly 12 in to 36 in, on top of the structural slab, so the finished floor sits that much higher and the clear height above the racks shrinks by the same amount unless the building was built taller to absorb it. On a slab the racks sit on the structural floor and the full height above is available for overhead busway, tray, and cooling.
Overhead distribution needs that height. Layered cable tray, busway, containment, and overhead cooling all hang from or near the ceiling, and TIA-942 calls for clearances between the tray layers and above them. A slab hall trades the under-floor void for the overhead void, and the overhead void is where the modern distribution lives.
For a building with a fixed floor-to-floor height, this is a real design lever. Spend the height on a deep plenum and you lose it overhead. Skip the plenum and you have room to run everything above the racks. On a tall new build it matters less. On a retrofit into an existing shell it can decide the floor approach by itself.
Access and maintenance
Access flips between the two designs, and overhead generally wins on a working floor. Under a raised floor you reach the power, the cabling, and the air path by lifting tiles, and the work happens on your knees in a void that is often full of legacy cable. Pull a tile in a live cold aisle and you also open a hole in the airflow plan and a trip hazard in the aisle. Overhead, the busway taps and the tray are in sight and in reach from a lift or a ladder, and changing a feed does not mean opening the floor.
The raised floor does keep one advantage: it hides the mess. A clean tile floor with everything below it reads tidier than a ceiling full of tray and busway, and some operators still prefer it for that. That is an aesthetic and a cleanliness call, not a capacity one.
The cleaning picture cuts the other way over time. The under-floor void collects dust and abandoned cable and is hard to clean without disrupting the airflow, while a slab floor is a surface you can see and clean. For a hall that will run for years, the maintainability of overhead and slab is part of why operators are choosing it.
First cost and life cost
On first cost, the slab is cheaper, because you are not buying and installing a whole floor system. A raised access floor is panels, pedestals, stringers, the ramp, the seals, and the labor to set it level and bonded, and it adds the structural design to carry the load through the understructure. Skip it and that line item goes away. Equinix and others have reported finding no compelling cost case for raised floor in new construction.
The cost that hides is the life cost. A raised floor needs its leakage managed, its tiles maintained, its cutouts sealed, and its understructure kept tight, for as long as the hall runs. Strengthening it later for a denser load means heavier panels and more pedestals, a real and disruptive expense. A slab does not carry those.
There is a case where the raised floor still earns its cost, and it is the hall that genuinely runs on under-floor air at a moderate density it can cool. There the plenum is doing a job the slab would have to do another way. Outside that case, the cost argument runs with the slab, and it runs harder every time the density goes up.
Can you retrofit a raised floor for higher density?
You can retrofit a raised-floor hall for higher density, but there is a ceiling on how far the plenum will take you, and past it the room has to change. The first moves are airflow management: seal the cutouts, blank the racks, contain the aisles, and place the tiles to the load. Those buy real capacity on a leaky floor and are cheap relative to the gain. Beyond that you can add cooling that does not depend on the plenum, in-row units or rear-door heat exchangers, on top of the existing floor.
The hybrid is common and it works when it is designed. Keep the raised floor for the moderate-density rows and the cable management, and bring overhead or in-row cooling to the high-density rows the plenum cannot feed. The risk is doing it by accident, dropping a dense row onto a floor planned for even air and stranding the cooling on the wrong side of it.
The hard limit is the load. A floor rated for yesterday's racks does not become a high-density floor by improving its airflow. If the new racks exceed the panel and pedestal ratings, the airflow retrofit does not save it, and the load rating guide is where that line gets drawn. Know which limit you are hitting, cooling or weight, before you spend on the wrong fix.
Grounding and bonding on each floor
Both floors get bonded, and the raised floor adds a grounding system the slab does not. A raised access floor is commonly tied together with a signal reference grid, an under-floor mesh of copper bonded to the building ground that ties the pedestals, the panels where required, and the equipment to a common reference. The metal floor structure is bonded so it cannot float at a different potential than the gear sitting on it.
On a slab the equipment grounding and bonding still happens, but there is no floor structure to bond, so the grounding follows the racks, the busway, and the building steel rather than a floor grid. The work is real on both. The raised floor is the one that adds the floor itself as a thing that has to be bonded and kept bonded as panels come and go.
The grounding and bonding rules follow the NEC, NFPA 70, and the project's grounding design, and the under-floor reference grid is detailed by topic in the load-rating and acceptance work. Confirm the bonding scheme against the design, because a raised floor with broken or missing bonds is a safety and a noise problem at once.
Water and leak detection under the floor
Water and a raised floor are a bad combination, and leak detection is how you live with it. A leak above the floor, a condensate line, a chilled-water connection, a liquid-cooling fitting, runs down through the cutouts into the plenum and pools on the structural slab below the tile, hidden, where it can reach the power and the cabling before anyone sees it. Under-floor leak detection, a sensing cable or spot sensors run on the slab below the raised floor, exists because you cannot see the water until you lift a tile.
On a slab design the floor is the slab, so a leak shows on the surface where staff and detection both find it faster. There is no void for the water to hide in. With liquid cooling spreading, this matters more, because there is more fluid in the room and more places for it to escape.
Leak detection is a design item either way, but the raised floor adds the under-floor case, water on the slab beneath the floor you are standing on. Design the detection to the cooling and the floor, and run it where the water will actually go. The detail is covered by topic with the cooling and the acceptance work. The design point is that the plenum is one more place a leak can hide.
Do data centers still use raised floors?
Data centers still use raised floors, but fewer new ones do, and the choice now turns on density and cooling. A raised floor still fits a hall that runs on under-floor air at a moderate density it can actually cool, where the plenum is doing useful work and the rack weights stay inside a rated floor. Plenty of running halls and many enterprise and smaller rooms are exactly that, and they are not wrong to be on tile.
Slab plus overhead fits the high-density and AI hall, the heavy and liquid-cooled rack, and the build that wants the flexibility and the access of overhead distribution. When the cooling goes liquid or in-row, the weight climbs, and the room is new construction, the slab is the default and the raised floor has to justify itself.
The decision is made early and it is hard to reverse, because the floor drives where the power, the cooling, and the cabling run. So make it on the real numbers: the density per rack, the cooling method, the rack weight, and the building height. The layout guide covers how that choice sits in the building zoning. The honest answer to whether the raised floor is dead is no. It is just no longer the default it was.
The hyperscale and AI move to slab
The hyperscale and AI build is where the shift is sharpest and most visible. These halls run racks pulling into the tens and hundreds of kilowatts, cooled by direct-to-chip liquid and in-row, weighing thousands of pounds each, deployed fast and at scale. Every one of those facts points away from a raised floor: no plenum needed, weight the slab handles better, and a build schedule that does not want to install and commission a floor system it will not use.
So the pattern on new AI capacity is slab, overhead busway and tray, and liquid cooling, with the raised floor reserved for the parts of the estate that still run on air. This is recent and it is fast. A hall designed for even, moderate density does not become an AI hall by swapping racks. The cooling, the power, and the structure all resize, and the floor decision resizes with them.
The hyperscalers set the direction the rest of the market follows, so the slab-plus-overhead pattern is showing up well below hyperscale as densities climb. It is the timely version of the same rule this whole comparison runs on: the cooling and the density drive the floor, and both are moving the same way.
The smaller and edge data center
The smaller and edge data center is the case where the raised floor often still makes sense, because the loads are modest and the build is small. An edge site, a small enterprise room, or a regional node typically runs lower-density, air-cooled racks where under-floor air at a moderate level is enough and the rack weights stay well inside a standard floor. The plenum does real work and the cost is small at that scale.
The counter-pressure is that even small rooms are getting denser, and an edge site dropping in a few high-density nodes hits the same wall a big hall does. At that point in-row cooling and overhead distribution start to make sense even in a small room, and the floor choice follows the same logic, just at a smaller size.
The rule does not change with the building. Size the floor to the density and the cooling, not to habit or to what the last room used. A small room with a light, air-cooled load is a fine place for a raised floor. A small room about to take a dense AI node is not, and it is worth knowing which one you are building before the floor goes in.
Commissioning the floor on each design
The floor gets commissioned either way, and what you prove depends on which floor it is. On a raised floor you verify the structure and the air: the panels and understructure match the rated configuration, the pedestals are plumb and bonded, the stringers are tight, the floor is level, the plenum holds pressure, the cutouts are sealed, the bonding is continuous, and where the rating is in doubt you run a field load test. The acceptance packet and the load test are the subject of the load rating guide.
On a slab there is no floor system to accept, so the commissioning shifts to the overhead distribution and the cooling: the busway and tray installed and supported, the overhead or in-row cooling proven to hold setpoint under load, the leak detection working, and the rack anchorage to the slab confirmed. The structure to prove is the building slab carrying the point loads, which is a structural check rather than a floor-panel one.
Either way the room is proven under load before the IT arrives, commonly with load banks, and the order matters: prove the support before you trust it with the gear. The acceptance detail for the floor lives in the load rating guide. The design point is that the floor choice changes what the commissioning has to prove, not whether it has to prove it.
What to document
A floor decision that no one wrote down is a decision the next team re-litigates. Capture which floor the hall uses and why, the density and cooling it was sized for, and the consequences that ride on the choice, so the basis of the decision survives the people who made it.
Record the floor type and the rationale, the design density per rack, the cooling method, the rack weight cases and the floor rating or the slab capacity, the air-delivery and the cabling and power routing, the leakage and leak-detection approach, the grounding scheme, and the seismic basis. Where the hall is hybrid, record which rows are which and why. The table below lays the comparison out the way a basis-of-design should.
| Factor | Raised access floor | Slab plus overhead |
|---|---|---|
| Cold-air delivery | Under-floor plenum, perforated tiles | Overhead, in-row, or contained supply |
| Power and cabling | Under the floor, floor PDUs and whips | Overhead busway and cable tray |
| Heavy rack weight | Limited by panel and pedestal rating | Carried by the structural slab |
| Best density fit | Lower to moderate, air-cooled | High density, liquid and in-row |
| Air leakage | Plenum leaks at cutouts, needs sealing | No plenum to leak |
| Ceiling height | Plenum eats clear height | Full height available overhead |
| Access | Lift tiles, work below | Overhead, in sight and reach |
| First cost | Higher, full floor system | Lower, no floor system |
| Seismic path | Through pedestals and stringers | Direct rack-to-slab anchorage |
| Water leak risk | Hides on slab under the floor | Visible on the floor surface |
Common mistakes
- Speccing a raised floor for a high-density load the plenum cannot actually cool.
- Letting the under-floor plenum leak at cutouts and gaps, throwing away the cold air it exists to deliver.
- Loading a rated raised floor past its panel and pedestal rating, especially the rolling load during move-in.
- Letting under-floor cabling and power accumulate until it chokes the airflow the plenum is supposed to move.
- Running liquid cooling over an open plenum with no under-floor leak detection on the slab below.
- Picking a raised floor by default without running the density, cooling, weight, and height analysis first.
- Dropping a high-density row onto a floor planned for even air and stranding the cooling on one side.
- Spending the building's clear height on a deep plenum the hall does not need, then having no room overhead.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
TIA-942 is the data center infrastructure standard, and it frames both the raised-floor and the overhead approaches: the access-floor guidance, the recommended overhead cable tray scheme with the layers separated and clearances held, and the white-space layout the floor sits in. The most recent edition is ANSI/TIA-942-C, published in 2024, but confirm the edition the project cites, because the section organization changes between editions.
The raised floor's own structural standard is the CISCA Recommended Test Procedures for Access Floors, which define the concentrated, rolling, ultimate, pedestal, and impact load test methods. CISCA defines the methods, not pass/fail values; the rated numbers come from the specification and the manufacturer's data. The full load and acceptance picture is the subject of the raised-floor load rating guide. ASHRAE Technical Committee 9.9 publishes the thermal guidelines the cooling and the white-space environment are designed to, which is the same reference the air-delivery choice is held to.
The structure under and around the floor follows the building code. Gravity and seismic design for the access floor as a nonstructural component, and for a slab carrying the equipment, follow the IBC and the referenced ASCE 7. The electrical installation, the grounding, and the working clearances follow the NEC, NFPA 70. Across all of it, the basis of design and the adopted code edition with local amendments control the specific numbers, so verify them against the design and the jurisdiction rather than carrying a remembered figure.
Units, terms, and abbreviations
The floor decision crosses a few trades and a few names, so the same idea reads differently across a structural drawing, a mechanical spec, and a cabling plan. Pin the term to the design before you act on it.
- Raised access floor
- A grid of removable panels on adjustable pedestals above the structural slab, with the void below used as a plenum
- Slab-on-grade
- The structural concrete floor the racks sit on directly, with power, cooling, and cabling run overhead
- Plenum
- The sealed under-floor void that holds and distributes pressurized cold supply air
- Pedestal / stringer
- The vertical column and the horizontal member that carry an access floor's load and lock its grid
- Perforated tile
- A floor panel with openings that lets plenum air up into the cold aisle
- Busway
- An overhead bus run with plug-in tap-off boxes that feeds rack power without new conduit
- Signal reference grid (SRG)
- An under-floor copper mesh bonded to building ground that ties the floor and gear to a common reference
- CDU
- Coolant distribution unit, the heavy interface between a facility loop and a liquid-cooled row
FAQ
Do data centers still use raised floors?
Yes, many existing and lower-density halls still run on raised access floors, and they suit a room cooled by under-floor air at a weight a rated floor carries. New high-density and AI builds increasingly skip the raised floor for slab plus overhead, so it is no longer the default it was for thirty years.
What is a raised floor plenum?
A raised floor plenum is the sealed void under an access floor that a computer room pressurizes with conditioned air. Perforated tiles in the cold aisle let that air up into the equipment intakes. The same void historically carried power and cabling too, which is why a leaking, congested plenum cools poorly.
Why are data centers moving to slab?
High-density and AI racks need more cooling than a floor plenum can push through tiles, and they weigh more than a raised floor carries economically. Liquid and in-row cooling do not need the plenum, the slab takes the weight directly, and overhead distribution is more flexible, so new builds lean to slab.
What is the difference between raised floor and overhead distribution?
A raised floor runs cold air, power, and cabling under the floor and delivers air up through perforated tiles. Overhead distribution runs the power on busway and the cabling on tray above the racks, with cooling from overhead or in-row units. Overhead keeps the air path clear and stays in sight and reach.
How deep is a data center raised floor plenum?
Field plenums commonly run from about 12 in to 36 in deep, with deeper voids on higher-airflow halls. Published guidance puts the usable range near 6 in to 30 in and recommends at least 18 in where airflow matters. Deeper helps the air and costs you clear height above the racks.
Can a raised floor handle a liquid-cooled AI rack?
Sometimes, if the floor is rated for the weight and the move-in rolling load, but it gets expensive and the plenum buys nothing once the cooling is liquid. A flooded coolant distribution unit alone can weigh three tons. Most new liquid-cooled halls go slab, which carries the weight directly. Confirm the rating first.
Does a slab data center cost less than a raised floor?
On first cost, usually yes, because there is no panel, pedestal, and stringer system to buy and install. Industry analysis has found no compelling cost advantage for raised floor in new construction. The raised floor also adds life cost: sealing leakage, maintaining tiles, and strengthening it later for denser loads.
Why does a raised floor plenum lose so much cold air?
Cold air leaks out of the plenum at every cable cutout, pedestal gap, and unsealed tile edge, dropping the static pressure that feeds the tiles you want. Studies have tied roughly half a room's cooling capacity to unsealed cutouts in bad cases. Sealing cutouts with brushed grommets cuts that bypass airflow sharply.
Is a raised floor or slab better for a high-density data center?
For high density, slab plus overhead is usually better: it carries the heavy racks directly, suits liquid and in-row cooling that does not need a plenum, and keeps the air path clear. A raised floor can work if rated for the load and the room runs on under-floor air, but it rarely wins at high density.
What do I check before choosing raised floor or slab?
Run four numbers: the design density per rack, the cooling method, the wet rack weight with its move-in rolling load, and the building's clear height. Under-floor air at moderate density and weight favors a raised floor; liquid cooling, heavy racks, and overhead distribution favor slab. Decide early, because the floor drives every route.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.